Direct-coupled driver for Mach-Zehnder optical modulators

ABSTRACT

An optical modulator device directly-coupled to a driver circuit device. The optical modulator device can include a transmission line electrically coupled to an internal VDD, a first electrode electrically coupled to the transmission line, a second electrode electrically coupled to the first electrode and the transmission line. A wave guide can be operably coupled to the first and second electrodes, and a driver circuit device can be directly coupled to the transmission line and the first and second electrodes. This optical modulator and the driver circuit device can be configured without back termination.

CROSS-REFERENCES TO RELATED APPLICATIONS

This non-provisional patent application is a continuation of and claimspriority to U.S. Non-provisional patent application Ser. No. 13/831,076,filed on Mar. 14, 2013, which claims priority from U.S. ProvisionalPatent Application No. 61/706,217, filed Sep. 27, 2012, which isincorporated by reference herein for all purposes.

BACKGROUND OF THE INVENTION

The present invention is directed to data communication system andmethods.

Over the last few decades, the use of communication networks exploded.In the early days Internet, popular applications were limited to emails,bulletin board, and mostly informational and text-based web pagesurfing, and the amount of data transferred was usually relativelysmall. Today, Internet and mobile applications demand a huge amount ofbandwidth for transferring photo, video, music, and other multimediafiles. For example, a social network like Facebook processes more than500 TB of data daily. With such high demands on data and data transfer,existing data communication systems need to be improved to address theseneeds.

Over the past, there have been many types of communication systems andmethods. Unfortunately, they have been inadequate for variousapplications. Therefore, improved communication systems and methods aredesired.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to data communication system andmethods. More specifically, various embodiments of the present inventionprovide a direct coupled driver device for an optical modulator device,which can be a Mach-Zehnder Modulator (MZM) and the like. The driverdevice can be configured with an optical modulator within an opticalnetwork for high-speed signal transfer.

The present driver is used for optical communications links. It producesthe electrical signal that drives an optical modulator device. Theoptical modulator transfers variations of the electrical signal intocorresponding modulation of an optical signal parameter such asamplitude, phase, polarization or a combination of these, in order toconvey information on an optical wave such as one traveling through anoptical fiber. The electrical “modulator driver” circuit is an importantelectronic component since all of the data to be conveyed optically hasto pass through this electrical device, which typically requires it tooperate at the highest speed or data rate of any electrical component inthe communication system. In addition, the driver must produce anelectrical amplitude that is typically large relative to other datasignals in the system, due to the weakness of the electro-opticaleffects that are used in high-speed optical modulators. This makes themodulator driver a major contributor to power dissipation in the system,and puts a premium on techniques to reduce this power.

In an embodiment, the optical modulator device can include signalconductor electrodes connected to electrical transmission lines. Forexample, there can be an upper signal conductor electrode and a lowersignal conductor electrode, the upper signal conductor electrode and thelower rectangle being the similar electrode that is the return path forthis current. These two conductors could form a microstrip line, acoplanar waveguide (CPW), parallel plates, or some other transmissionline. The electric field from this electrical “RF” signal isconcentrated between these plates, where it interacts on the opticalwaveguide (shown in dotted lines) to produce the modulation effect inthe modulator material where the “RF” and optical fields overlap. Thetransmission lines can include a horizontal “CPW” transmission line orvertical “coplanar strips”, and the like. The electric field existsbetween the signal + and signal − electrodes for a “differential”structure, or between a signal electrode and ground for a “single ended”structure.

A feature of the present is that there is no back-termination R_(T).Therefore, the load R seen by the driver is just the load resistanceR_(L). R_(L) serves the same function in all examples: it is thetermination to the transmission lines inside the optical modulator. Theback-termination is generally used so that the driver has a goodimpedance match to a 50 ohm line, which is the usual way of connecting adriver and a modulator since they are traditionally designed as separateitems. However, the line between them may be long, multi-sectioned, orinvolve vias or connectors, etc., all of which can add reflections. Ifthe driver does not have a back termination, these reflections canbounce back and degrade the signal. If the driver is designed to beinterfaced directly to the modulator, as here, this problem can beeliminated; the modulator termination/load resistor may also be used asan input point for the driver power supply (V_(DD)).

A simplified schematic of the direct coupled driver includes the basicfunctioning of a bias control loop. The goal of the bias control loop isto keep a DC voltage equal to V_(MOD) applied across the modulatorelectrodes at all times. This is useful in semiconductor-based MZmodulators in which the electro-optic modulation effect occurs or ismaximized at a specific DC bias; for example, where the electrodecomprises a reverse-biased diode. The V_(MOD) voltage is the resultingDC voltage across each electrode. For the typical case of a DC-balanceddata signal, V_(MOD) can be considered to be the average DC voltage onVrf+ and Vrf− (e.g. the common-mode voltage at the driver outputterminals) minus the DC voltage on the opposite terminals of theserespective MZM electrodes. So, as the DC voltages at the Vrf+ and Vrf−pads vary due to supply voltage V_(DD) changing, or drive currentchanging, etc., the control loop keeps the DC voltage across theelectrodes constant. Thus, V_(MOD) serves as the bias point for theoptical modulator function, with the AC modulation voltage riding ontop. In practice there are a number of op-amp circuits that wouldperform this function.

In an embodiment, the present invention provides an optical modulatordevice directly-coupled to a driver circuit device. The opticalmodulator device can include a transmission line electrically coupled toan internal VDD, a first electrode electrically coupled to thetransmission line, a second electrode electrically coupled to the firstelectrode and the transmission line. A wave guide can be operablycoupled to the first and second electrodes, and a driver circuit devicecan be directly coupled to the transmission line and the first andsecond electrodes. This optical modulator and the driver circuit devicecan be configured without back termination.

It is to be appreciated that embodiments of the present inventionprovide numerous benefits and advantages over existing techniques. In anembodiment, the electrical driver is directly coupled to the modulatordevice or MZM, which can include a wire-bonded or flip-chippedapplication directly to a photonic circuit without transmission lineinterconnects. The driver can receive its power supply for the outputstage through the modulator termination resistors. This configurationcan eliminate the need for back termination at the driver, which savespower (e.g. >40%). Also, the need for bias tees or DC blocks betweendriver and modulator can be eliminated, which saves area (e.g. >100%).Furthermore, this configuration can allow a versatile drive capability,since the exact impedance of the modulator load is not critical tosignal quality. The DC bias of the modulator can also be configured witha bias control loop. There are many other benefits as well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram illustrating a leaf-spine architectureaccording to an embodiment of the present invention.

FIG. 2 is a simplified diagram illustrating the form factor of acommunication device according to an embodiment of the presentinvention.

FIG. 3A is a simplified diagram illustrating a communication interfaceaccording to an embodiment of the present invention.

FIG. 3B is a simplified diagram illustrating a segmented opticalmodulator according to an embodiment of the present invention.

FIG. 4 is simplified diagram of a conventional differential ordual-drive modulator.

FIG. 5 is a simplified diagram of a conventional single-ended modulator.

FIG. 6 is a simplified diagram of a conventional differential AC coupledmodulator.

FIG. 7 is a simplified diagram of a conventional single ended modulatorwith bias tee.

FIG. 8 is a simplified diagram of a direct coupled modulator accordingto an embodiment of the present invention.

FIG. 9 is a simplified diagram of a direct coupled modulator accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to data communication system andmethods. More specifically, various embodiments of the present inventionprovide a direct coupled driver device for an optical modulator device,which can be a Mach-Zehnder Modulator (MZM) and the like. The driverdevice can be configured with an optical modulator within an opticalnetwork for high-speed signal transfer.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented, but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without necessarily being limitedto these specific details. In other instances, well-known structures anddevices are shown in block diagram form, rather than in detail, in orderto avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, clockwise and counter clockwise have been used forconvenience purposes only and are not intended to imply any particularfixed direction. Instead, they are used to reflect relative locationsand/or directions between various portions of an object.

In the last decades, with advent of cloud computing and data center, theneeds for network servers have evolved. For example, the three-levelconfiguration that have been used for a long time is no longer adequateor suitable, as distributed applications require flatter networkarchitectures, where server virtualization that allows servers tooperate in parallel. For example, multiple servers can be used togetherto perform a requested task. For multiple servers to work in parallel,it is often imperative for them to be share large amount of informationamong themselves quickly, as opposed to having data going back forththrough multiple layers of network architecture (e.g., network switches,etc.).

Leaf-spine type of network architecture is provided to better allowservers to work in parallel and move data quickly among servers,offering high bandwidth and low latencies. Typically, a leaf-spinenetwork architecture uses a top-of-rack switch that can directly accessinto server nodes and links back to a set of non-blocking spine switchesthat have enough bandwidth to allow for clusters of servers to be linkedto one another and share large amount of data.

In a typical leaf-spine network today, gigabits of data are shared amongservers. In certain network architectures, network servers on the samelevel have certain peer links for data sharing. Unfortunately, thebandwidth for this type of set up is often inadequate. It is to beappreciated that embodiments of the present invention utilizes PAM(e.g., PAM8, PAM12, PAM16, etc.) in leaf-spine architecture that allowslarge amount (up terabytes of data at the spine level) of data to betransferred via optical network.

FIG. 1 is a simplified diagram illustrating a leaf-spine architecture100 according to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The leaf-spine architecture100 comprises servers 120, leaf switches 110, and spine switches 103. Itis to be appreciated that depending on the need and specificapplication, the number and arrangement of the servers and switches maybe changed. As shown in FIG. 1, each server may be connected to morethan one leaf switch. For example, server 121 is connected to leafswitches 111 and 112. Similarly, server 122 is connected to leafswitches 111 and 112, and so is server 123. In an exemplary embodiment,server 121 is connected to the leaf switch 111 via optical communicationlink utilizing pulse amplitude modulation (PAM). PAM2, PAM4, PAM8,PAM12, PAM16, and/or other variations of PAM may also be used inconjunction with optical communication links in various embodiments ofthe present invention. The bandwidth of the optical communication linkbetween the server 121 and leaf switch 111 can be over 10 gigabits/s.Each leaf switch, such as leaf switch 111, may be connected to 10 ormore servers. In one implementation, a leaf switch has a bandwidth of atleast 100 gigabits/s.

In a specific embodiment, a leaf switch comprises a receiver deviceconfigured to receive four communication channels, and each of thechannels is capable of transferring incoming data at 25 gigabits/s andconfigured as a PAM-2 format. Similarly, a server (e.g. server 121)comprises communication interface that is configured to transmit andreceive at 100 gigabits/sec (e.g., four channels at 25 gigabits/s perchannel), and is compatible with the communication interface of the leafswitches. The spine switches, similarly, comprise communicationinterfaces for transmitting and receiving data in PAM format. The spineswitches may have a large number of communication channels toaccommodate a large number of leaf switches, each of which providesswitching for a large number of servers.

The leaf switches are connected to spine switches. As shown in FIG. 1,each leaf switch is connected to spine switches 101 and 102. Forexample, leaf switch 111 is connected to the spine switch 101 and 102,and so are leaf switches 113 and 114. In a specific embodiment, each ofthe spine switches is configured with a bandwidth of 3.2 terabytes/s,which is big enough to communicate 32 optical communication links at 100gigabits/s each. Depending on the specific implementation, otherconfiguration and bandwidth are possible as well.

The servers, through the architecture 100 shown in FIG. 1, cancommunicate with one another efficiently with a high bandwidth. Opticalcommunication links are used between servers and leaf switches, and alsobetween leaf switches and spine switches, and PAM utilized for opticalnetwork communication.

It is to be appreciated that the PAM communication interfaces describedabove can be implemented in accordance with today communicationstandards form factors. In addition, afforded by high efficiency level,network transceivers according to embodiments of the present inventioncan have much lower power consumption and smaller form factor comparedto conventional devices. FIG. 2 is a simplified diagram illustrating theform factor of a communication device according to an embodiment of thepresent invention. Today, C form-factor pluggable (CFP) standard iswidely adapted for gigabit network systems. Conventionalelectrical-connection based CFP transceivers often use 10×10 gigabits/slines to achieve high bandwidth. With optical connection, CFPtransceivers can utilize 10×10 gigabits/s configuration, 4×25 gigabits/sconfiguration, or others. It is to be appreciated that by utilizingoptical communication link and PAM format, a transceiver according tothe present invention can have a much smaller form factor than CFP andCFP2 as shown. In various embodiments, communication interfacesaccording to the invention can have a form factor of CFP4 or QSFP, whichare much smaller in size than the CFP. In addition to smaller formfactors, the power consumption of communication interfaces according tothe present invention can be much smaller. In a specific embodiment,with the form factor of QSFP, the power consumption can be as low asabout 3 W, which is about ¼ that of convention transceivers with CFPform factor. The reduce level of power consumption helps save energy atdata centers, where thousands (sometimes millions) of thesecommunication devices are deployed.

FIG. 3A is a simplified diagram illustrating a communication interface300 according to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The communication interface300 includes transmitter module 310 and a receiver module 320. Thetransmitter module 310 comprises a receiver 311, encoder 312, and PAMmodulation driver 313.

In an embodiment, the communication interface 300 is configured toreceive incoming data at through four channels, where each channel isconfigured at 25 gigabits/s and configured as a PAM-2 format. Using thetransmitter module 310, modulator 316, and the laser 314, thecommunication interface 300 processes data received at 25 gigabits/sfrom each of the four incoming channels, and transmits PAM modulatedoptical data stream at a bandwidth of 100 gigabits/s. It is to beappreciated that other bandwidths are possible as well, such as 40 Gbps,400 Gbps, and/or others.

As shown the transmitter module 310 receives 4 channels of data. It isto be appreciated that other variants of pulse-amplitude modulation(e.g., PAM4, PAM8, PAM12, PAM16, etc), in addition to PAM-2 format, maybe used as well. The transmitter module 310 comprises functional block311, which includes a clock data recovery (CDR) circuit configured toreceive the incoming data from the four communication channels. Invarious embodiments, the functional block 311 further comprisesmultiplexer for combining 4 channels for data. For example, data fromthe 4 channels as shown are from the PCE-e interface 350. For example,the interface 350 is connected to one or more processors. In a specificembodiment, two 2:1 multiplexers are employed in the functional block311. For example, the data received from the four channels arehigh-speed data streams that are not accompanied by clock signals. Thereceiver 311 comprises, among other things, a clock signal that isassociated with a predetermined frequency reference value. In variousembodiments, the receiver 311 is configured to utilize a phase-lockedloop (PLL) to align the received data.

The transmitter module 310 further comprises an encoder 312. As shown inFIG. 3, the encoder 312 comprises a forward error correction (FEC)encoder. Among other things, the encoder 312 provides error detectionand/or correction as needed. For example, the data received is in aPAM-2 format as described above. The received data comprises redundancy(e.g., one or more redundant bits) helps the encoder 312 to detecterrors. In a specific embodiment, low-density parity check (LDPC) codesare used. The encoder 312 is configured to encode data received fromfour channels as shown to generate a data stream that can be transmittedthrough optical communication link at a bandwidth 100 gigabits/s (e.g.,combining 4 channels of 25 gigabits/s data). For example, each receivedis in the PAM-2 format, and the encoded data stream is a combination offour data channels and is in PAM-8 format. Data encoding and errorcorrection are used under PAM format. The PAM formats as used in theembodiments of the present invention are further described below.

The PAM modulation driver 313 is configured to drive data stream encodedby the encoder 312. In various embodiments, the receiver 311, encoder312, and the modulation driver 313 are integrated and part of thetransmitter module 310.

The PAM modulator 316 is configured to modulate signals from thetransmitter module 310, and convert the received electrical signal tooptical signal using the laser 314. For example, the modulator 316generates optical signals at a transmission rate of 100 gigabits persecond. It is to be appreciated that other rate are possible as well,such as 40 Gbps, 400 Gbps, or others. The optical signals aretransmitted in a PAM format (e.g., PAM-8 format, PAM12, PAM 16, etc.).In various embodiments, the laser 314 comprises a distributed feedback(DFB) laser. Depending on the application, other types of lasertechnology may be used as well, as such vertical cavity surface emittinglaser (VCSEL) and others.

FIG. 3B is a simplified diagram illustrating a segmented opticalmodulator according to an embodiment of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. For example, modulated PAMsignals modulated for transmission over optical communication links.

Now referring back to FIG. 3A. The communication interface 300 isconfigured for both receiving and transmitting signals. A receivermodule 320 comprise a photo detector 321 that converts incoming datasignal in an optical format converts the optical signal to an electricalsignal. In various embodiments, the photo detector 321 comprises indiumgallium arsenide material. For example, the photo detector 321 can be asemiconductor-based photodiode, such as p-n photodiodes, p-i-nphotodiodes, avalanche photodiodes, or others. The photo detector 321 iscoupled with an amplifier 322. In various embodiments, the amplifiercomprises a linear transimpedance amplifier (TIA). It is to beappreciated by using TIA, long-range multi-mode (LRM) at high bandwidth(e.g., 100 Gb/s or even larger) can be supposed. For example, the TIAhelps compensate for optical dispersion in electrical domain usingelectrical dispersion compensation (EDC). In certain embodiments, theamplifier 322 also includes a limiting amplifier. The amplifier 322 isused to produce a signal in the electrical domain from the incomingoptical signal. In certain embodiments, further signal processing suchas clock recovery from data (CDR) performed by a phase-locked loop mayalso be applied before the data is passed on.

The amplified data signal from the amplifier 322 is processed by theanalog to digital converter (ADC) 323. In a specific embodiment, the ADC323 can be a baud rate ADC. For example, the ADC is configured toconvert the amplified signal into a digital signal formatted into a 100gigabit per second signal in a PAM format. The functional block 324 isconfigured to process the 100 Gb/s data stream and encode it into fourat streams at 25 Gb/s each. For example, the incoming optical datastream received by the photo detector 321 is in PAM-8 format at abandwidth of 100 Gb/s, and at block 324 four data streams in PAM-2format is generated at a bandwidth of 25 Gb/s. The four data streams aretransmitted by the transmitter 325 over 4 communication channels at 25Gb/s.

It is to be appreciated that there can be many variations to theembodiments described in FIG. 3. For example, different number ofchannels (e.g., 4, 8, 16, etc.) and different bandwidth (e.g., 10 Gb/s,40 Gb/s, 100 Gb/s, 400 Gb/s, 3.2 Tb/s, etc.) can be used as well,depending on the application (e.g., server, leaf switch, spine switch,etc.).

In an embodiment, the present invention provides a driver device for andoptical modulator device. The present driver is used for opticalcommunications links. It produces the electrical signal that drives theoptical modulator device. The optical modulator transfers variations ofthe electrical signal into corresponding modulation of an optical signalparameter such as amplitude, phase, polarization or a combination ofthese, in order to convey information on an optical wave such as onetraveling through an optical fiber. The electrical “modulator driver”circuit is an important electronic component since all of the data to beconveyed optically has to pass through this electrical device, whichtypically requires it to operate at the highest speed or data rate ofany electrical component in the communication system. In addition, thedriver must produce an electrical amplitude that is typically largerelative to other data signals in the system, due to the weakness of theelectro-optical effects that are used in high-speed optical modulators.This makes the modulator driver a major contributor to power dissipationin the system, and puts a premium on techniques to reduce this power.FIGS. 4-7 describe some conventional modulators for reference.

FIG. 4 is simplified diagram of a conventional differential ordual-drive modulator. As shown, device 400 includes an optical waveguide410 and signal conductor electrodes 420 connected to electricaltransmission lines. The upper rectangle shown is one signal conductorelectrode and the lower rectangle is a similar electrode that is forreturn path for this current. The electric field from this electrical“RF” signal is concentrated between these plates or conductors 420,where it interacts on the optical waveguide 410 to produce themodulation effect in the modulator material where the “RF” and opticalfields overlap. In this case, the electric field exists between thesignal + and signal − electrodes, hence a “differential” structure.

FIG. 5 is a simplified diagram of a conventional single-ended modulator.As shown, device 500 includes an optical waveguide 510 and signalconductor electrodes 520 connected to electrical transmission lines.Similar to device 400 of FIG. 4, the electric field from this electrical“RF” signal is concentrated between these plates or conductors 420,where it interacts on the optical waveguide 410 to produce themodulation effect in the modulator material where the “RF” and opticalfields overlap. This device 500 shows a modulator with a horizontal“CPW” transmission line compared to the modulator with vertical“coplanar strips” as shown in FIG. 4. In this case, the electric fieldexists between the signal electrode and ground for the “single-ended”structure.

FIG. 6 is a simplified diagram of a conventional differential AC coupledmodulator. As shown, device 600 includes an optical waveguide 610 andsignal conductor electrodes 620 connected to electrical transmissionlines and a driver circuit device 630. The modulator of device 600 is adifferential modulator, similar to device 400 of FIG. 4, which is ACcoupled. A disadvantage of this configuration is that with the driverbeing coupled to the modulator or

MZM, the driver requires a back termination at the driver to absorbreflections from interconnect and bias elements. Transmission lineinterconnects and DC blocking capacitors are needed. These allcontribute to a larger structure that is inefficient in power and sizedue to conventional design barriers (standardization forces a 50-ohmtransmission line interface with no DC interaction allowed). Here, thedifferential modulator is coupled to a differential operationalamplifier driver circuit device 630.

Graph 601 shows a current-voltage characteristic graph for the device600. Both the AC load line and DC load lines are shown. As shown in FIG.6, V_(pp,SE) is the single-ended peak-to-peak voltage, I is the averagecurrent, V is the supply voltage, and the R is resistance. Table 602shows a comparison of supply power values (Supplypower=I*V=(2V_(pp,SE)/R)(1.5V_(pp,SE)+2V_(min))) given different outputswing values and minimum voltage values.

FIG. 7 is a simplified diagram of a conventional single ended modulatorwith bias tee. As shown, device 700 includes an optical waveguide 710and signal conductor electrodes 720 connected to electrical transmissionlines, a driver circuit device 730, and a bias tee 740. The modulator ofdevice 700 is a single ended modulator, similar to device 500 of FIG. 5.A disadvantage of this configuration is that with the driver beingcoupled to the modulator or MZM, the driver requires a back terminationat the driver to absorb reflections from interconnect and bias elements.Transmission line interconnects and inductor/capacitor bias-tees areneeded. These all contribute to a larger structure that is inefficientin power and size due to conventional design barriers (standardizationforces a 50-ohm transmission line interface with no DC interactionallowed). Here, the single ended modulator is coupled to a single endedamplifier driver circuit device 730.

Graph 701 shows a current-voltage characteristic graph for the device700. Both the AC load line and DC load lines are shown. Table 602 showsa comparison of supply power values (Supplypower=I*V=(V_(pp)/R)(0.5V_(pp)+V_(min))) given different output swingvalues and minimum voltage values.

FIG. 8 is a simplified diagram of a direct coupled modulator accordingto an embodiment of the present invention. As shown, device 800 includesan optical waveguide 810 and signal conductor electrodes 820 connectedto electrical transmission lines and a driver circuit device 830. Thesignal conductor electrodes 820 can form a microstrip line, a coplanarwaveguide (CPW), parallel plates, or some other transmission line, orthe like. The electric field from this electrical “RF” signal isconcentrated between these plates, where it interacts on the opticalwaveguide (shown in dotted lines) to produce the modulation effect inthe modulator material where the “RF” and optical fields overlap. Themodulator of device 800 is a differential modulator, similar to device400 of FIG. 4.

FIG. 8 shows an embodiment of the present invention. One difference indevice 800 compared to the previous figures is that there is noback-termination R_(T). Therefore, the load R seen by the driver is justthe load resistance R_(L). R_(L) serves the same function in allexamples: it is the termination to the transmission lines inside theoptical modulator. The back-termination is generally used so that thedriver has a good impedance match to a 50 ohm line, which is the usualway of connecting a driver and a modulator since they are traditionallydesigned as separate items. However, the line between them may be long,multi-sectioned, or involve vias or connectors, etc., all of which canadd reflections. If the driver does not have a back termination, thesereflections can bounce back and degrade the signal. If the driver isdesigned to be interfaced directly to the modulator, as shown here, thisproblem can be eliminated; the modulator termination/load resistor mayalso be used as an input point for the driver power supply (V_(DD)), asshown in FIG. 8.

Graph 801 shows a current-voltage characteristic graph for the device800. Both the AC load line and DC load lines are shown. Table 802 showsa comparison of supply power values (Supplypower=I*V=(2V_(pp,SE)/R)(1.5V_(pp,SE)+2V_(min))) given different outputswing values and minimum voltage values.

It is to be appreciated that embodiments of the present inventionprovide numerous benefits and advantages over existing techniques. In anembodiment, the electrical driver is directly coupled to the modulatordevice or MZM, which can include a wire-bonded or flip-chippedapplication directly to a photonic circuit without transmission lineinterconnects. The driver can receive its power supply for the outputstage through the modulator termination resistors. This configurationcan eliminate the need for back termination at the driver, which savespower (e.g. >40%). Also, the need for bias tees or DC blocks betweendriver and modulator can be eliminated, which saves area (e.g. >100%).Furthermore, this configuration can allow a versatile drive capability,since the exact impedance of the modulator load is not critical tosignal quality. The DC bias of the modulator can also be configured witha bias control loop. There are many other benefits as well.

FIG. 9 is a simplified diagram of a direct coupled modulator accordingto an embodiment of the present invention. As shown, device 900 includesan optical waveguide 910 and signal conductor electrodes 920 connectedto electrical transmission lines and a driver circuit device 930, whichcan have a modulator bias loop 940. A simplified schematic whichillustrates the basic functioning of a bias control loop is shown inFIG. 9. The goal of the bias control loop is to keep a DC voltage equalto V_(MOD) applied across the modulator electrodes at all times. This isuseful in semiconductor-based MZ modulators in which the electro-opticmodulation effect occurs or is maximized at a specific DC bias; forexample, where the electrode comprises a reverse-biased diode. TheV_(MOD) voltage is the resulting DC voltage across each electrode

For the typical case of a DC-balanced data signal, V_(MOD) can beconsidered to be the average DC voltage on Vrf+ and Vrf− (e.g. thecommon-mode voltage at the driver output terminals) minus the DC voltageon the opposite terminals of these respective MZM electrodes. So, as theDC voltages at the Vrf+ and Vrf− pads vary due to supply voltage V_(DD)changing, or drive current changing, etc., the control loop keeps the DCvoltage across the electrodes constant. Thus, V_(MOD) serves as the biaspoint for the optical modulator function, with the AC modulation voltageriding on top. In practice there are a number of op-amp circuits thatwould perform this function.

In an embodiment, the present invention provides an optical modulatordevice directly-coupled to a driver circuit device. The opticalmodulator device can include a transmission line electrically coupled toan internal VDD, a first electrode electrically coupled to thetransmission line, a second electrode electrically coupled to the firstelectrode and the transmission line. A wave guide can be operablycoupled to the first and second electrodes, which can be reverse-biaseddiodes and the like. The transmission line and the first and secondelectrodes can be configured as a microstrip line, a coplanar waveguide(CPW), or a parallel plate transmission line, or the like. A drivercircuit device can be directly coupled to the transmission line and thefirst and second electrodes. This optical modulator and the drivercircuit device can be configured without back termination. The opticalmodulator can be a Mach-Zehnder Modulator (MZM).

In a specific embodiment, the driver circuit device can include a driverop-amp circuit electrically coupled to the transmission line, and a biasloop circuit electrically coupled to the first and second electrodes andthe driver op-amp circuit. The bias loop circuit device can include abuffer op-amp, a voltage sources, and a high impedance node. The bufferop-amp can be electrically coupled to the voltage source and the highimpedance node. The high impedance node can be electrically coupled tothe driver op-amp circuit and the transmission line. The voltage sourcecan be electrically coupled to the first and second electrodes, and canbe configured to provide the electrodes with a bias voltage.

In a specific embodiment, this optical modulator directly coupled to adriver circuit device can be provided within an optical network system.This system can include a receiver device configured to receive fourcommunication channels. Each of the channels can be capable oftransferring incoming data at 25 GPS. The system can also include aclock data recovery circuit configured to receive the incoming data fromthe four communication channels. An encoder can be provided to formatthe incoming data from these four channels. The driver circuit devicecan be configured to drive the encoded incoming data, and the modulatorcan include a PAM modulator configured for receiving the encodedincoming data and transferring an outgoing signal at a rate of at least40 Gbps per second using a PAM form with the incoming data beingconfigured as a PAM-2 format.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. A data communication method comprising:electrically coupling a transmission line to an internal VDD in anoptical modulator device; electrically coupling a first electrode to thetransmission line; electrically coupling a second electrode to the firstelectrode and the transmission line; operably coupling a wave guide tothe first and second electrodes; directly coupling a driver circuitdevice to the transmission line and the first and second electrodes; thedriver circuit producing an electrical signal including variations tothe first and second electrodes; and the first and second electrodestransferring the variations into corresponding modulations of aparameter of an optical signal communicated along the transmission line.2. The data communication method of claim 1 wherein directly couplingthe driver circuit device comprises electrically coupling a driverop-amp circuit to the transmission line, and electrically coupling abias loop circuit to the first and second electrodes and the driverop-amp circuit.
 3. The data communication method of claim 2 wherein thebias loop circuit device comprises a buffer op-amp, a voltage source,and a high impedance node, the method comprising electrically couplingthe buffer op-amp to the voltage source and the high impedance node,electrically coupling the high impedance node to the driver op-ampcircuit and the transmission line, and electrically coupling the voltagesource to the first and second electrodes.
 4. The data communicationmethod of claim 1 comprising configuring the optical modulator deviceand the driver circuit device without back termination.
 5. The datacommunication method of claim 1 wherein the optical modulator devicecomprises a Mach-Zehnder Modulator (MZM).
 6. The data communicationmethod of claim 1 wherein the first and second electrodes comprisereverse-biased diodes.
 7. The data communication method of claim 1wherein the transmission line and the first and second electrodes areconfigured as a microstrip line, a coplanar waveguide (CPW), or aparallel plate transmission line.
 8. A data communication methodcomprising: directly coupling a driver op-amp circuit to an opticalmodulator device; and coupling a bias control loop circuit to the driverop-amp circuit and the optical modulator device, the bias control loopcircuit including a buffer op-amp, a voltage source, and a highimpedance node.
 9. The data communication method of claim 8 comprisingelectrically coupling the buffer op-amp to the voltage source and thehigh impedance node, electrically coupling the high impedance node tothe driver op-amp circuit and the optical modulator, and electricallycoupling the voltage source to the optical modulator.
 10. The of datacommunication method of claim 8 comprising configuring the opticalmodulator device and the driver circuit device without back termination.11. The data communication method of claim 8 wherein the opticalmodulator device comprises a differential modulator device, a singleended modulator device, or a Mach-Zehnder modulator (MZM) device. 12.The data communication method of claim 8 wherein the optical modulatordevice comprises a pair of modulator electrodes, the method comprisingconfiguring the bias control loop circuit to provide a bias voltage tothe modulator electrodes.
 13. A data communication method comprising:configuring a receiver device to receive four communication channels,each of the channels being capable of transferring incoming data at 25GPS; configuring a clock data recovery circuit to receive the incomingdata from the four communication channels; an encoder formatting theincoming data from the four channel communications; providing an opticalmodulator device by, electrically coupling a transmission line to aninternal VDD, electrically coupling a first electrode to thetransmission line, electrically coupling a second electrode to the firstelectrode and the transmission line, and operably coupling a wave guideto the first and second electrodes; and directly coupling a drivercircuit device to the transmission line and the first and secondelectrodes, the driver device being configured to drive the encodedincoming data; the driver circuit producing an electrical signalincluding variations to the first and second electrodes; and the firstand second electrodes transferring the variations into correspondingmodulations of a parameter of an optical signal communicated along thetransmission line.
 14. The data communication method of claim 13 whereindirectly coupling the driver circuit comprises electrically coupling adriver op-amp circuit to the transmission line, and electricallycoupling a bias loop circuit to the first and second electrodes and thedriver op-amp circuit.
 15. The data communication method of claim 14wherein electrically coupling the bias loop circuit device compriseselectrically coupling a high impedance node to the driver op-amp circuitand the transmission line, electrically coupling a buffer op-amp to thehigh impedance node, and electrically coupling a voltage source to thefirst and second electrodes and the buffer op-amp.
 16. The datacommunication method of claim 13 comprising configuring the opticalmodulator device and the driver circuit device without back termination.17. The data communication method of claim 13 wherein the opticalmodulator device comprises a Mach-Zehnder Modulator (MZM).
 18. The datacommunication method of claim 13 wherein the first and second electrodescomprise reverse-biased diodes.
 19. The data communication method ofclaim 13 comprising configuring the transmission line and the first andsecond electrodes as a microstrip line, a coplanar waveguide (CPW), or aparallel plate transmission line.
 20. The data communication method ofclaim 13 wherein the optical modulator device comprises a PAM modulatorconfigured to receiving the encoded incoming data and transferring anoutgoing signal at a rate of at least 40 Gbps per second using a PAMform, the incoming data being configured as a PAM-2 format.